I only looked at briefly, but I saw nothing on there that says athiests have killed more.
I refuse to believe that atheists have killed more people that Hitler, the Crusades, Muslims, and Aztecs have as a basis of "faith".
MAO killed over 40,000,000.
Stalin killed over 40,000,000.
Pol Pot killed over 2,000,000.
In the other communist revolution in Vietnam and Korea over 1 million died in this conflicts as well.
All were athiests
40 million were killed by the mongols.
Crusade by the way don't even make the list with around 1.5 million, Christians and Muslims killed.